1. Field of the Invention
The present invention generally relates to the production of gypsum board materials and, more specifically, the invention relates to the manufacture of gypsum wallboard utilizing an extrusion technique to prepare a gypsum core.
2. Description of the Related Art
A common method of constructing walls and ceilings includes the use of inorganic wallboard panels or sheets, such as gypsum wallboard, often referred to simply as “wallboard” or “drywall.” Wallboard can be formulated for interior, exterior, and wet applications. The use of wallboard, as opposed to conventional boards made from wet plaster methods, is desirable because the installation of wallboard is ordinarily less costly and less cumbersome when compared to the installation of conventional plaster walls.
Walls and ceilings made with gypsum wallboard panels typically are constructed by securing, e.g., with nails or screws, the wallboard panels to structural members, such as vertically- and horizontally-oriented pieces of steel or wood often referred to as “studs.” Because wallboard typically is supplied in standard-sized sheets or panels, when forming a wall from the sheets, there will generally be a number of joints between adjacent sheets. In most wallboard construction, these joints typically are filled and coated with an adhesive material called a joint compound so that the wall will have a smooth finish similar to that obtained with conventional plaster walls.
Generally, wallboard is produced by enclosing a core of an aqueous slurry of calcined gypsum and other materials between two large sheets of board cover paper. Various types of cover paper are known in the art. After the gypsum slurry has set (i.e., reacted with water present in the aqueous slurry) and dried, the formed sheet is cut into standard sizes. Methods for the production of gypsum wallboard generally are described, for example, by Michelsen, T. “Building Materials (Survey),” Encyclopedia of Chemical Technology, (1992 4th ed.), vol. 21, pp. 621–24, TP9. E685, the disclosure of which is hereby incorporated herein by reference.
Gypsum wallboard is manufactured utilizing commercial processes that are capable of operation under continuous, high-speed conditions. A conventional process for manufacturing the core composition of gypsum wallboard initially includes the premixing of dry ingredients in a high speed mixing apparatus. The dry ingredients can include calcium sulfate hemihydrate (stucco), an accelerator, and an antidesiccant (e.g., starch). The dry ingredients are mixed together with a “wet” (aqueous) portion of the core composition in a pin mixer apparatus. The wet portion can include a first component, commonly referred to as a “paper pulp solution,” that includes a mixture of water, paper pulp, and, optionally, one or more fluidity-increasing agents, and a set retarder. The paper pulp solution provides a major portion of the water that forms the gypsum slurry of the core composition. A second wet component can include a mixture of the aforementioned strengthening agent, foam, and other conventional additives, if desired. Together, the aforementioned dry and wet portions comprise an aqueous gypsum slurry that eventually forms a gypsum wallboard core.
A major ingredient of the gypsum wallboard core is calcium sulfate hemihydrate, commonly referred to as “calcined gypsum,” “stucco,” or “plaster of Paris.” Stucco has a number of desirable physical properties including, but not limited to, its fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. Typically, stucco is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). The drying step in the manufacture of stucco includes passing crude gypsum rock through a rotary kiln to remove any free moisture present in the rock from rain or snow, for example. The dried rock then is passed through a roller mill (or impact mill types of pulverizers), wherein the rock is ground or comminuted to a desired fineness. The degree of comminution is determined by the ultimate use. The dried, fine-ground gypsum can be referred to as “land plaster” regardless of its intended use. The land plaster is used as feed to calcination processes for conversion to stucco.
The calcination (or dehydration) step in the manufacture of stucco is performed by heating the land plaster, and generally can be described by the following chemical equation which shows that heating calcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco) and water vapor:CaSO4.2H2O+heat#CaSO4.½H2O+1 ½H2O.
This calcination process step is performed in a “calciner,” of which there are several types known by those of skill in the art.
Uncalcined calcium sulfate (i.e.,and plaster) is the “stable” form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction is actually a reversal of the above-described chemical reaction performed during the calcination step. The setting reaction proceeds according to the following chemical equation which shows that the calcium sulfate hemihydrate is rehydrated to its dehydrate state:CaSO4.½H2O+1 ½H2O#CaSO4.2H2O+heat.
The actual time required to complete the setting reaction generally depends upon the type of calciner and the type of gypsum rock that are used to produce the gypsum, and can be controlled within certain limits by the use of additives such as retarders, set accelerators, and/or stabilizers, for example. Generally, the rehydration time period can be in a range of about two minutes to about eight hours depending on the quantity of retarders, set accelerators, and/or stabilizers present.
After the aqueous gypsum slurry is prepared, the slurry and other desired ingredients are continuously deposited to form a gypsum wallboard core (hereinafter “wallboard core” or “core”) slurry between two continuously-supplied moving sheets of cover paper. The two cover sheets comprise a pre-folded face paper and a backing paper. As the slurry is deposited onto the face paper, the backing paper is brought down atop the deposited core slurry and bonded to the prefolded edges of the face paper. The whole assembly then is sized for thickness utilizing a roller bar or forming plate. The deposited core slurry is then allowed to set between the two cover sheets, thereby forming a board. The continuously-produced board is cut into panels of a desired length, which are vertically-stacked, and then passed through a drying kiln where excess water is removed from the board to form a strong, dry, and rigid building material.
The cover sheets used in the process typically are multi-ply paper manufactured from re-pulped newspapers. The face paper has an unsized inner ply which contacts the core slurry such that gypsum crystals can grow up to (or into) the inner ply-this, along with the starch present in the slurry, is the principal form of bonding between the core slurry and the cover sheet. The middle plies are sized and an outer ply is more heavily sized and treated to control absorption of paints and sealers. The backing paper is also a similarly constructed multi-ply sheet. Both cover sheets must have sufficient permeability to allow for water vapor to pass therethrough during the downstream board drying step(s).
Standardized sheets (or panels) of wallboard typically are about four feet (about 1.22 meters) wide and about 8 feet to about 16 feet (about 2.4 meters to about 4.9 meters) in length. Sheets typically are available in thicknesses varying in a range of about ¼ inch to about one inch (about 0.6 centimeters to about 2.6 centimeters).
In order to provide satisfactory strength, commercially-available gypsum wallboard generally requires a density of about 1600 to about 1700 pounds (about 726 to about 772 kilograms) per thousand square feet (lbs/MSF) of one-half inch board. Heavy or high-density gypsum wallboards are more costly and difficult to manufacture, transport, store, and manually install at job sites, compared to lighter or low-density boards. It is possible to formulate wallboard having reduced densities through the inclusion of lightweight fillers and foams, for example. Often, however, where wallboard is formulated to have a density less than about 1600 lbs/MSF of one-half inch board, the resulting strength is unacceptable for commercial sale. Because high-density or heavy gypsum wallboard generally is not desirable, various attempts have been made to reduce board weight and density without sacrificing board strength. However, while lighter and less dense wallboard products can be produced, many of the wallboard products may be of a quality ill-suited for commercial use.
Morris et al. U.S. Pat. No. 5,482,551 discloses an extrudable composition for use in production of articles for building and construction made of about 45–85 weight percent calcium sulfate dihydrate and a filler to control density.
In view of the foregoing, it would be desirable to produce high strength gypsum wallboard having weights and densities generally equal to or slightly less than that produced by conventional methods. Reduced weight and density boards, however, should meet industry standards and have strengths similar to, or greater than, conventional wallboard. Such wallboard also should be able to be manufactured using high-speed manufacturing apparatus and not suffer from other negative side-effects. For example, such high-strength wallboard should be able to set and dry within a reasonable period of time.
Further, the use of conventional ingredients in the preparation of the aqueous slurry can cause fouling and undesired plugging of mixers and tubing used to prepare and convey, respectively, the aqueous slurry onto the paper cover sheet(s). For example, admixing set accelerators into the slurry in an upstream mixer, such as a pin mixer, can cause the slurry to begin setting (i.e., calcine or harden) before its deployment onto the cover sheet(s). Thus, it would be desirable to produce gypsum wallboard utilizing processes that do not require certain ingredients in the gypsum slurry which could cause fouling or premature setting of the slurry, or impart other undesired effects.